It is well established that assessing pulmonary arterial pressure (PAP) is useful in diagnosing pulmonary disease such as pulmonary hypertension. PAP, however, may remain relatively unchanged following clinical treatment to address pulmonary hypertension. Thus, while useful as an indicator of the presence of pulmonary disease, PAP alone may not be practical for evaluating improvement and patient response to treatment. For this reason, additional pulmonary circulation information is desired.
Total pulmonary vascular resistance (TPR) provides information about the flowrate and pressure in the pulmonary vasculature. TPR is a ratio of the PAP to flowrate. Unlike PAP, TPR changes to some degree in response to treatment and can therefore be used to help assess the efficacy or degree of the treatment effect that eventually results in patient improvement.
Various approaches are available to obtain pulmonary information. For example, a passive approach is disclosed in Patent Pub. No. 2001/0039383 to Mohler. In this application, a sensor assembly is provided that detects heart sounds. The sensor is placed in contact with the skin, and is connected with a signal processing means. Pressure information is measured based on acoustic wave signatures arising from the heart. See also U.S. Pat. No. 6,368,283 to Xu et al.
A more invasive approach involves implanting a medical device for sensing the absolute and barometric pressures as disclosed in U.S. Pat. No. 6,024,704 to Meador et al. The implantable sensor described in Meador includes two leads. One lead is positioned in the subcutaneous layer of the skin for measuring the barometric pressure and a second lead is positioned in the right ventricle of the heart for measuring pressure therein. The contents of the above teachings are incorporated by reference in their entirety and may be optionally combined with the methods and devices described herein.
Despite the results of the above mentioned measurement techniques, an invasive catheterization procedure is still considered by many physicians to be the gold standard to confirm pulmonary hypertension and for determining the PAP. In a right heart catheterization, a catheter is inserted through the pulmonary arteries and into the right ventricle of the heart. Once positioned in the right ventricle, the pulmonary pressure is measured directly using a pressure sensor. However, this procedure requires penetration of the vasculature, and requires significant surgical intervention. As such, the procedure carries an undesirable amount of inconvenience, cost, and risk to the patient.
A more convenient, less invasive approach is desired for obtaining accurate pulmonary circulation information.